1. Field of the Invention
The present invention relates to an automatic focusing adjusting device for use in an optical instrument such as a camera or the like and, in particular, to an automatic focusing adjusting device ideal for use in a camera which is capable of performing an operational processing on the phase difference detection to be executed when the focusing of an image pickup optical system is detected, by means of an analog signal processing.
2. Description of the Related Art
Conventionally, an automatic focusing detecting device, which is used in an automatic focusing adjusting device for use in an optical instrument such as a camera or the like, has such structure as shown in FIG. 17. In this structure, a taking lens 1 is disposed in the left-most position, that is, upstream in FIG. 17. Downstream of the taking lens 1, there are arranged an image pickup equivalent surface 2, a condenser lens 3, a separator lens 4, and a phase difference detecting device in this order.
The phase difference detecting device comprises two line sensors 5, 6 which are adapted to receive a pair of object image to be focused by the separator lens 4 and to convert the light images into electric signals, and a process circuit 7 which checks focusing conditions based on the electric signals that are generated according to the distributions of the light intensity in the respective pixels of the line sensors 5, 6.
The images focused on the line sensors 5, 6 are closer to an optical axis 8 in a forwardly focused state in which the image of the object is positioned forwardly of the image pickup equivalent surface 2, are farther from the optical axis 8 in a rearwardly focused state, and are situated at a predetermined position midway between the forwardly and rearwardly focused states in a focused state. For this reason, the process circuit 7 checks for the focused state by detecting the position of the focused image with respect to the optical axis 8 in accordance with the electric signals (distance measurement information) that are generated from the line sensors 5 and 6, respectively. Conventionally, in order to detect the position of the images focused on the line sensors 5 and 6, a phase difference detection method is used. According to this method, an operation is carried out based on the following equation (1) to find the correlation operation values of a pair of focused images on the line sensors 5 and 6, such operation is continued until the smallest correlation value is found, and the focused state is checked based on an amount of relative movement (a phase difference) of the positions of the focused images with respect to the position of the focused images in focus. ##EQU1## where, l is an integral number 1 through 9 and represents the above-mentioned amount of relative movement.
Also, when l=1, no shift operation is performed, and when l.gtoreq.2, the shift operation can be carried out.
Here, B(K) represents electrical signals which are output in a time series manner from the respective pixels of the line sensor 5, and R (K+l-1) expresses electrical signals which are output in a time series manner from the respective pixels of the line sensor 6. Thus, if the above-mentioned equation (1) is operated each time 1 is caused to vary from 1 to 9, then the correlation operation values H(1), H(2), ... H(9) can be obtained. For example, it is previously set that the images focused on the line sensors are in the focused state when the correlation operation value H(5) becomes the smallest value. Then, if any correlation operation value at a position shifted from the correlation operation value H(5) becomes the smallest value, then the amount of such shifting, that is, the phase difference to l=5 can be detected as the amount of out-of-focus (the amount of de-focusing).
Now, in FIG. 18, there is shown the structure of the conventional process circuit 7. In this structure, the analog electric signals B(K), R(K) that are generated by the respective pixels of the line sensors 5, 6 are converted by an A/D converter 9 into digital data of, for example, 8 bits, and the digital data are once stored through a microcomputer 10 into a RAM (Random Access Memory). After then, the above-mentioned equation is operated based on these digital data.
By the way, in the automatic focusing adjusting device, if the number of pixels of a line sensor (which is referred to as a range of distance measurement) that are used to detect focusing (that is, to measure distances) is constant, then the following problems arise. In other words, for example, when photographing by use of a camera, if the distance measurement range of the line sensor is too wide, then various objects existing in far and near places are output from the line sensor as the distance measurement information, with the result that the object to be essentially distance measured cannot be selected. On the other hand, if the distance measurement range is too narrow, then, for example, when taking a picture of the face of a character in a close-up manner by means of telephotography, there is produced a monotonous picture in which no light or shade can be seen in the various parts of the character's face.
Also, in a plurality of optical instruments such as cameras or the like which have different image pickup optical systems and different screen sizes from one another, if the distance measurement range of the line sensor is constant, then a ratio of are to be occupied by the line sensor in the screen is cause to vary to a great extent, so that similar problems as in the above mentioned case will arise.
In view of this, in the conventional automatic focusing adjusting device, in order to vary the distance measurement range of the line sensor according to the objects to be photographed, the photoelectric conversion element of the line sensor belonging to the distance measurement range is addressed by use of control means and is then activated. However, this results in the increased loads of the software and thus the cost of the automatic focusing adjusting device is increased, which provides a big problem to be solved.